He began World War I with this unit, but transferred to aviation in November 1916.

Between 9:00 and 9:15 AM on 14 September, Müller shot down three Spad XIIIs from the American 13th Aero Squadron .

The aviator changed his last name to Garrelt to obviate confusion in their mail delivery.

[2] During World War II, Müller served on the staff of Luftflotte 3 of the Luftwaffe.

[2] Post World War II, he designed steam locomotives for Hanomag and Henschel.